In a liquid electrophotographic (LEP) printer, a latent image is created on the surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment dispersed in an insulating carrier liquid. The toners are selectively attracted to the photoconductor surface either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode, and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For LEP laser printers, the preferred embodiment is that the photoconductor and toner have the same polarity.
A sheet of paper is given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, which may be in the form of a continuous belt or a rotating drum, pulling the toner from the photoconductor surface onto the paper still in the pattern of the image developed from the photoconductor surface.
A recent demand for color laser printers has prompted designers to use liquid toners with pigment components dispersed in a liquid carrier medium. With liquid toners, however, there is a need to remove the liquid carrier medium from the photoconductor surface after the toner has been applied to it. The photoconductor surface requires carrier and excess toner removal so that it will not transfer the liquid carrier to the paper (or other intermediate transfer medium) in the image transfer step. Also, in this way the liquid carrier may be recovered for recycle and reuse in the developer system, providing economy in terms of printing supplies, and eliminating environmental and health concerns from disposal of excess liquid carrier medium.
LEP printer technology also requires that back-plated and film-formed toner be cleaned from various components in the machine, specifically the developer roller and an image compacting (rigidizing) squeegee roller or metering roller. Back-plated toner is that toner repelled to the developer roller from the photoconductor in the negative of the image that is to be developed on the photoconductor. Film-formed toner is the toner that has begun to dry and, consequently, formed a film.
It has been suggested that cleaning of LEP components be accomplished through the use of a foam roller in U.S. Pat. No. 5,300,990, incorporated in full herein by reference. In this configuration, a foam roller 16 is counter-rotated against the developer roller 14 to clean off the back-plated toner. The foam roller is then wrung out by a wringer bar roller 22 (not shown) located near the bottom of the foam roller and in constant contact with the foam roller.
Several problems result from this configuration. First, the foam roller tends to retain toner solids. This takes the toner out of the printing cycle and reduces the life of a toner cartridge. Second, if the roller is left idle long enough, the solids form a film (film-form) and dry out and the roller adheres to any other adjacent, contacting rollers. Third, when the developer is not rotating (not printing) for a long period of time, the wringer bar creates a large compression set in the foam roller, rendering it useless for cleaning in that area until it has rotated a sufficient number of times to relax the compression set. Depending on the print process, this may or may not occur within one print cycle.
Generally, the problem of toner solids build-up in the foam roller has only been corrected by foam roller replacement when the build-up is too great. However, changing foam rollers is often tedious and expensive.
A prior solution to the problem of compression set has been to let the developer rotate at all times or rotate occasionally to reduce the chance of compression set. Rotation could also conceivably be done during every start-up cycle. However, any system which requires the developer to be rotated during idle periods or upon start-up will be more costly and complex.
There is, therefore, a need for a means of reducing or eliminating foam roller compression set and also a need for a foam roller configuration that permits maximum removal of liquid toner. Accordingly, objects of the present invention are to correct these problems with an efficient yet inexpensive foam roller cleaning system.